Caprolactam is a widely used intermediate in the production of nylon-6 fibers, molding compounds and various plastic articles. Caprolactam is conventionally prepared by the reduction of phenol to cyclohexanone, oximation of cyclohexanone to cyclohexanone oxime and Beckmann rearrangement of cyclohexanone oxime to caprolactam. This method proceeds with high yields in all steps but employs hydroxylamine salts in various forms as the nitrogen source. Since preparation of a hydroxylamine salt solution from ammonia and other inorganic reagents can be difficult and expensive and can cause a large quantity of by-products, a need exists for a method of producing caprolactam and related amino acids without the use of hydroxylamine salts as intermediates.
The oxidative cleavage of phenol is known. Phenol can be oxidized successively to catechol (1,2-dihydroxybenzene), 1,2-benzoquinone and muconic acid monoesters. It has been reported that 4-tert-butyl-1,2-benzoquinone can be oxidized to a mixture of 3- and 4-tert-butylmuconic acid monomethyl esters by a copper(II) complex, pyridine cupric methoxy chloride in pyridine containing water. It has also been reported that phenol can be cleaved to cis,cis-muconic acid monomethyl ester in a system of pyridine cupric methoxy chloride complex and molecular oxygen.
It is also known that phenol, catechol and orthobenzoquinone can be oxidized with peracetic acid to cis,cis-muconic acid, which isomerizes, under certain conditions, to the cis,trans and trans,trans forms. The hydrogenation of muconic acid to adipic acid has also been reported. Adipic acid is commercially reacted with ammonia to form the corresponding diamide which is dehydrated and hydrogenated to hexamethylene diamine, the comonomer with adipic acid in nylon-66.
Copper-based materials are also known to catalyze the oxidative coupling of certain substituted phenols to diphenoquinones and biphenols as described, for example, in U.S. Pat. No. 4,096,190 to Rutledge (June 20, 1978). In certain examples of that patent, a copper salt and ammonia were suspended with stirring in water, a substituted phenol such as 2,6 xylenol (2,6-dimethyl phenol) was added and then oxygen flow was initiated. In example 12, the initial copper salt was cuprous chloride. It is not clear if, under such conditions, a copper(II)-ammonia reagent of the type described herein would be formed.